Backlighting help requested

Take a look at your shutter speed. Your camera wants to do 1/2500 to expose the scene without flash. However, when you turn on the flash, your shutter speed is limited to your max flash sync speed (1/250), which is 10x slower, blowing out your background.

When using the flash as a fill, try dropping down to ISO 100 and stopping down your aperture to let in less light, compensating for the increased light let in by your 1/250 shutter. Or try a ND filter to knock down your exposure.

Another approach would be to skip the flash and get a reflector to through more ambient light on the subject, but gear might not suit a location like a ship.